1. Field of the Invention
The present invention relates to a focusing error detection circuit which may be used with, for example, an optical pickup of an optical disk apparatus.
2. Description of the Prior Art
In a conventional apparatus for recording and/or reproducing information signals on or from an optical disk, an optical pickup device is used, in which a light beam from a semiconductor laser is collected by an objective lens, is irradiated on the recording tracks of an optical disk and the reflected return light from the disk is detected to effect the readout and/or writing of the information signals.
A conventional optical pickup device includes a built-in focusing control unit for controlling an objective lens such that the light beam collected by the objective lens is irradiated onto the recording tracks of the optical lens in a focused state. The focusing control unit performs focusing control in response to focusing error signals by driving a supporting unit, which supports the object lens and is movable in two directions, specifically, along the optical axis of the object lens and along the direction normal to the optical axis and which are both within a horizontal plane. A conventional supporting unit for supporting the object lens to move in the two mutually prependicular directions is shown for example in the U.S. Pat. No. 4,473,274. The focusing error signals are produced by detecting with a photodetector device the state of the reflected return light which has been once irradiated onto and reflected by the optical disk surface. A known device for detecting these focusing error signals, is a so-called focusing error detection circuit, operating in accordance with the astigmatic method for example, as disclosed in U.S. Pat. No. 4,023,033.
As shown in FIG. 1, in a known focusing error detection circuit 1 which employs the astigmatic system, laser light irradiated onto an optical disk, not shown, from an optical pickup, also not shown, and then reflected by the optical disk, to forms a return beam which impinges, as a light spot SP, on a four-segment photodetector 2.
When the return beam falls on the four-segment photodetector 2 in the form of the light spot SP, light reception currents S.sub.A, S.sub.B, S.sub.C and S.sub.D are generated as outputs from first to fourth photodetector segments 2A, 2B, 2C and 2D of the four-segment photodetector, respectively. The light reception currents S.sub.A, S.sub.B, S.sub.C and S.sub.D are supplied to the inverting inputs of first to fourth current-to-voltage converters 3A, 3B, 3C and 3D constituted by respective operational amplifiers.
The current to voltage converters 3A to 3D are provided with negative feedback paths including resistors R.sub.A1, R.sub.B1, R.sub.C1 and R.sub.D1, respectively, while having their non-inverting inputs grounded. The first to fourth light reception voltages V.sub.A, V.sub.B, V.sub.C and V.sub.D, converted from the light reception currents S.sub.A, S.sub.B, S.sub.C and S.sub.D, respectively, are outputted from the output terminals of the converters 3A, 3B, 3C, 3D, respectively.
Each of the first to fourth light reception voltages V.sub.A to V.sub.D is added to another light reception voltage. The first and the third light reception voltages V.sub.A and V.sub.C are transmitted through first and third resistor R.sub.A2 and R.sub.C2, each of equal resistance values, and added together before being entered into a non-inverting input of a subtractor 4 of an operational amplifier configuration. In the same manner, the second and the fourth light reception voltages V.sub.B and V.sub.D are transmitted through second and fourth resistor R.sub.B2 and R.sub.D2, each of equal resistance values, and added together before being entered into an inverting input of the subtractor 4.
The subtractor 4 has a negative feedback path, through a resistor R.sub.1 and has its non-inverting input grounded through resistor R.sub.2. A differential voltage, which is the difference between the sum voltage of the first and the third light reception voltages V.sub.A and V.sub.C and the sum voltage of the second and the fourth light reception voltages V.sub.B and V.sub.D, is obtained at the output of the subtractor 4. In this manner, the focusing error detection circuit 1 of an astigmatic system generates a focusing error signal S.sub.FE from the light reception currents S.sub.A to S.sub.D of the four-segment photodetector 2 according to the formula EQU S.sub.FE =(S.sub.A +S.sub.C)-(S.sub.B +S.sub.D) (1)
In order that the problems overcome by the present invention may be fully understood, the focusing error signal S.sub.FE and the RF signal S.sub.RF will be described with reference to FIGS. 2a, 2b and 2c.
In FIG. 2a, the focusing error signal S.sub.FE is shown to be generated so as to have a voltage waveform in the form of a letter S with respect to the distance between the optical disk and the objective lens of the optical pickup device.
The objective lens of the optical pickup device is usually controlled by a focusing servo device so that the detected focusing error signal S.sub.FE will be at a point of intersection P.sub.0 between the central portion of the S-shaped voltage waveform and the zero voltage, called a zero-crossing-point. At the zero-crossing-point, the laser light of the optical pickup device is irradiated on the optical disk in the just focused state or the correctly focused state.
In FIG. 2b, the focusing error detection circuit 1 is assuumed to be susceptible to DC drift due to changes in the operating temperature or the environmental temperature of the operational amplifiers of the substractor 4 and the current-to-voltage converters 3A to 3D. A DC offset proportional to the change in temperature shifts the focusing error signal S.sub.FE1 to cause fluctuations in the zero-crossing-point P.sub.1 and, hence, in the operating point of the focusing servo device.
The DC offset, which is proportional to the DC drift, becomes conspicuous in the event of significant changes in temperature, when the volume of the incident light impinging on the four-segment photodetector 2 is small, or when a broad bandwidth DC amplifier circuit is used as the operational amplifier of the subtractor 4.
FIG. 2c shows that the subtractor 4, when designed with a broad bandwidth DC amplifier circuit, generates as output an RF signal S.sub.RF. When exposed to significant changes in temperature, P.sub.0 on the RF signal S.sub.RF shifts on the curve from P.sub.0 to P.sub.1 and, accordingly, cannot be reproduced correctly.